Jervell and Lange-Nielsen syndrome (JLN) is an autosomal recessive form of long QT syndrome (LQT). In addition to QT interval prolongation, this disorder is associated with congenital deafness. JLN is rare, but affected individuals are susceptible to cardiac arrhythmias with a high incidence of sudden death and short life expectancy. The present invention is directed to a mutation in the KVLQT1 gene which results in Jervell and Lange-Nielsen syndrome and to probes and methods for diagnosing the presence of JLN. JLN is diagnosed in accordance with the present invention by analyzing the DNA sequence of the KVLQT1 gene of an individual to be tested and comparing the respective DNA sequence to the known DNA sequence of a normal KVLQT1 gene.
The publications and other materials used herein to illuminate the background of the invention or provide additional details respecting the practice, are incorporated by reference, and for convenience are respectively grouped in the appended l ist of References.
Cardiac arrhythmias are a common cause of morbidity and mortality, accounting for approximately 11% of all natural deaths (Kannel, 1987; Willich et al., 1987). In general, presymptomatic diagnosis and treatment of individuals with life-threatening ventricular tachyarrhythmias is poor, and in some cases medical management actually increases the risk of arrhythmia and death (Cardiac Arrhythmia Suppression Trial II Investigators, 1992). These factors make early detection of individuals at risk for cardiac arrhythmias and arrhythmia prevention high priorities.
Both genetic and acquired factors contribute to the risk of developing cardiac arrhythmias. Long QT syndrome (LQT) is an inherited cardiac arrhythmia that causes abrupt loss of consciousness, syncope, seizures and sudden death from ventricular tachyarrhythmias, specifically torsade de pointes and ventricular fibrillation (Ward, 1964; Romano, 1965; Schwartz et al., 1975; Moss et al., 1991). This disorder usually occurs in young, otherwise healthy individuals (Ward, 1964; Romano, 1965; Schwartz et al., 1975). Most LQT gene carriers manifest prolongation of the QT interval on electrocardiograms, a sign of abnormal cardiac repolarization (Vincent et al., 1992). The clinical features of LQT result from episodic cardiac arrhythmias, specifically repolarization-related ventricular tachyarrhythmias like torsade de pointes, named for the characteristic undulating nature of the electrocardiogram in this arrhythmia and ventricular fibrillation (Schwartz et al., 1975; Moss and McDonald, 1971). Torsade de pointes may degenerate into ventricular fibrillation, a particularly lethal arrhythmia. Although LQT is not a common diagnosis, ventricular arrhythmias are very common; more than 300,000 United States citizens die suddenly every year (Kannel, et al., 1987; Willich et al., 1987) and, in many cases, the underlying mechanism may be aberrant cardiac repolarization. LQT, therefore, provides a unique opportunity to study life-threatening cardiac arrhythmias at the molecular level.
Both inherited and acquired forms of LQT have been defined. Acquired LQT and secondary arrhythmias can result from cardiac ischemia, bradycardia and metabolic abnormalities such as low serum potassium or calcium concentration (Zipes, 1987). LQT can also result from treatment with certain medications, including antibiotics, antihistamines, general anesthetics, and, most commonly, antiarrhythmic medications (Zipes, 1987). Inherited forms of LQT can result from mutations in at least five different genes. In previous studies, LQT loci were mapped to chromosome 11p15.5 (KVLQT1 or LQT1) (Keating et al., 1991a; Keating et al., 1991b), 7q35-36 (HERG or LQT2), 3p21-24 (SCN5A or LQT3) (Jiang et al., 1994). Of these, the most common cause of inherited LQT is KVLQT1. Our data indicate that mutations in this gene are responsible for more than 50% of inherited LQT. Recently, a fourth LQT locus (LQT4) was mapped to 4q25-27 (Schott et al., 1995). Also, KCNE1 (LQT5) has been associated with long QT syndrome (Splawski et al., 1997b; Duggal et al., 1998). These genes encode ion channels involved in generation of the cardiac action potential. Mutations can lead to channel dysfunction and delayed myocellular repolarization. Because of regional heterogeneity of channel expression with the myocardium, the aberrant cardiac repolarization creates a substrate for arrhythmia. KVLQT1 and KCNE1 are also expressed in the inner ear (Neyroud et al., 1997; Vetter et al., 1996). We and others demonstrated that homozygous or compound heterozygous mutations in each of these genes can cause deafness and the severe cardiac phenotype of the Jervell and Lange-Nielsen syndrome (Neyroud et al., 1997; Splawski et al., 1997a; Schultze-Bahr et al., 1997; Tyson et al., 1997). Loss of functional channels in the ear apparently disrupts the production of endolymph, leading to deafness.
Autosomal dominant and autosomal recessive forms of this disorder have been reported. Autosomal recessive LQT (also known as Jervell and Lange-Nielsen syndrome) has been associated with congenital neural deafness; this form of LQT is rare (Jervell and Lange-Nielsen, 1957). Autosomal dominant LQT (Romano-Ward syndrome) is more common, and is not associated with other phenotypic abnormalities (Romano et al., 1963; Ward, 1964). A disorder very similar to inherited LQT can also be acquired, usually as a result of pharmacologic therapy (Schwartz et al., 1975; Zipes, 1987).
The data have implications for the mechanism of arrhythmias in LQT. Two hypotheses for LQT have previously been proposed (Schwartz et al., 1994). One suggests that a predominance of left autonomic innervation causes abnormal cardiac repolarization and arrhythmias. This hypothesis is supported by the finding that arrhythmias can be induced in dogs by removal of the right stellate ganglion. In addition, anecdotal evidence suggests that some LQT patients are effectively treated by .beta.-adrenergic blocking agents and by left stellate ganglionectomy (Schwartz et al., 1994). The second hypothesis for LQT-related arrhythmias suggests that mutations in cardiac-specific ion channel genes, or genes that modulate cardiac ion channels, cause delayed myocellular repolarization. Delayed myocellular repolarization could promote reactivation of L-type calcium channels, resulting in secondary depolarizations (January and Riddle, 1989). These secondary depolarizations are the likely cellular mechanism of torsade de pointes arrhythmias (Surawicz, 1989). This hypothesis is supported by the observation that pharmacologic block of potassium channels can induce QT prolongation and repolarization-related arrhythmias in humans and animal models (Antzelevitch and Sicouri, 1994). The discovery that one form of LQT results from mutations in a cardiac potassium channel gene supports the myocellular hypothesis.
In theory, mutations in a cardiac sodium channel gene could cause LQT. Voltage-gated sodium channels mediate rapid depolarization in ventricular myocytes, and also conduct a small current during the plateau phase of the action potential (Attwell et al., 1979). Subtle abnormalities of sodium channel function (e.g., delayed sodium channel inactivation or altered voltage-dependence of channel inactivation) could delay cardiac repolarization, leading to QT prolongation and arrhythmias. In 1992, Gellens and colleagues cloned and characterized a cardiac sodium channel gene, SCN5A (Gellens et al., 1992). The structure of this gene was similar to other, previously characterized sodium channels, encoding a large protein of 2016 amino acids. These channel proteins contain four homologous domains (DI-DIV), each of which contains six putative membrane spanning segments (S1-S6). SCN5A was recently mapped to chromosome 3p21, making it an excellent candidate gene for LQT3 (George et al., 1995), and this gene was then proved to be associated with LQT3 (Wang et al., 1995).
In 1994, Warmke and Ganetzky identified a novel human cDNA, human ether a-go-go related gene (HERG, Warmke and Ganetzky, 1994). HERG was localized to human chromosome 7 by PCR analysis of a somatic cell hybrid panel (Warmke and Ganetzky, 1994) making it a candidate for LQT2. It has predicted amino acid sequence homology to potassium channels. HERG was isolated from a hippocampal cDNA library by homology to the Drosophila ether a-go-go gene (eag), which encodes a calcium-modulated potassium channel (Bruggemann et al., 1993). HERG is not the human homolog of eag, however, sharing only .about.50% amino acid sequence homology. HERG has been shown to be associated with LQT2 (Curran et al., 1995).
LQT1 was found to bc linked with the gene KVLQT1 (Q. Wang et al., 1996). Sixteen families with mutations in KVLQT1 were identified and characterized and it was shown that in all sixteen families there was complete linkage between LQT1 and KVLQT1. KVLQT1 was mapped to chromosome 11p15.5 making it a candidate gene for LQT1. KVLQT1 encodes a protein with structural characteristics of potassium channels, and expression of the gene as measured by Northern blot analysis demonstrated that KVLQT1 is most strongly expressed in the heart. One intragenic deletion and ten different missense mutations which cause LQT were identified in KVLQT1. These data define KVLQT1 as a novel cardiac potassium channel gene and show that mutations in this gene cause susceptibility to ventricular tachyarrhythmias and sudden death.
It was known that one component of the I.sub.Ks channel is minK, a 130 amino acid protein with a single putative transmembrane domain (Takumi et al., 1988; Goldstein and Miller, 1991; Hausdorffet al., 1991; Takumi et al., 1991; Busch et al., 1992; Wang and Goldstein, 1995; KW Wang et al., 1996). The size and structure of this protein made it unlikely that minK alone forms functional channels (Attali et al., 1993; Lesage et al., 1993). It has been shown that KVLQT1 and minK coassemble to form the cardiac I.sub.Ks potassium channel (Sanguinetti et al., 1996; Barhanin et al., 1996). I.sub.Ks dysfunction is a cause of cardiac arrhythmia. It was later shown that mutations in KCNE1 (which encodes minK) also can result in LQT (Splawski et al., 1997b).
In 1957, Jervell and Lange-Nielsen reported a syndrome associated with congenital sensory deafness and prolonged QT interval in four children of a Norwegian family (Jervell and Lange-Nielsen, 1957). The affected children had multiple syncopal episodes and three died suddenly at ages 4, 5 and 9. Since 1957, other examples of long QT syndrome (LQT) associated with deafness (Jervell and Lange-Nielsen syndrome or JLN) have been described (Fraser et al., 1964; Jervell et al., 1966; Tesson et al., 1996). In all cases the apparent mode of inheritance was autosomal recessive. This syndrome is rare (estimated incidence of 1.6 to 6 per million) (Fraser et al., 1964). Affected individuals are susceptible to recurrent syncope with a high incidence of sudden death and short life expectancy. Syncope results from torsade de pointes ventricular tachycardia and ventricular fibrillation (Till et al., 1988; Holland, 1993).
Romano-Ward syndrome is the autosomal dominant form of LQT and is not associated with deafness or other phenotypic abnormalities (Romano et al., 1963; Ward, 1964). The incidence of Romano-Ward is higher than JLN, but affected individuals generally have milder symptoms (Moss et al., 1985; Moss et al., 1991).
We hypothesized that JLN results from mutations affecting both alleles of an autosomal dominant LQT gene. It is here demonstrated that homozygous mutation of KVLQT1 causes JLN. Other family members also had LQT with an autosomal dominant pattern of inheritance but these individuals had normal hearing and were heterozygotes.